1. Field of the Invention
The present invention relates to a process for the fabrication of a capacitor having a structure wherein a thin film layer of dielectric material and an electrode layer are stacked on a metal foil, and more particularly to a process for the fabrication of a capacitor that is slimmed down in its entirety in such a way as to be built in a board (interconnecting board) for electronic packages, etc., and that is to be buried in the board.
2. Explanation of the Prior Art
A thin-film capacitor comprising a thin-film dielectric layer and an electrode layer provided on a metal foil is known as a buried type capacitor that is well fit for being incorporated in a board. In that capacitor, the metal foil takes a role of supporting a capacitor formed thereon, and works as an electrode for the completed capacitor as well. For this reason, the thin-film capacitor comprising a thin-film dielectric layer and an electrode layer provided on that metal foil is simpler in structure, lower in fabrication cost, and smaller in overall thickness than a thin-film capacitor comprising an electrode layer, a thin-film dielectric layer and an electrode layer provided on an insulating substrate: it is best suited as a buried type capacitor for interconnecting boards.
The prior art, for instance, JP(A)2000-164460 discloses a thin-film capacitor comprising a dielectric material on a metal foil selected from copper, nickel and so on.
Further, JP(A)2003-526880 discloses that a dielectric thin film having a perovskite crystal structure such as lead zironate titanate has a higher dielectric constant, lower dielectric losses and more limited leak currents, and may be fabricated by techniques such as sol-gel processes or sputtering processes, forming a PZT thin film on a foil such as one formed of brass, platinum, titanium or stainless steel.
Furthermore, JP(A)2001-210789 discloses a process wherein, to form on a member of low heat resistance a capacitor equipped with a dielectric thin film having a perovskite crystal structure formed at high temperatures, a noble metal is formed on a high heat-resistant substrate such as a silicon substrate, then a capacitor structure is formed, then the capacitor structure is bonded onto a separately made member of low heat resistance, and finally the capacitor structure is separated off the high heat-resistant substrate such as the silicon substrate.
For the metal foil, however, its surface must be fully smoothened so as to prevent the capacitor from shorts, and this requirement is found to become one reason for high costs involved. Further, when a thin substrate is used to make the whole capacitor thin, there is much difficulty in handling it, which is found to become a leading reason for lower yields.
When a copper foil that is considered most preferable because of low electric resistance is used as the metal foil for burying purposes, it is found to be incapable of being fully sintered at high temperatures in an oxidizing atmosphere because copper is susceptible of oxidization and has a low melting point. This is also true even when use is made of the same composition material as a perovskite ceramics dielectric material that is a high dielectric constant material. It is still impossible to obtain a device of large enough capacity due to inadequate crystallization, or there is a problem with large leak currents. When use is made of a nickel foil that is relatively less susceptible of oxidization although its electric resistance is somewhat high, too, there is still a problem with oxidization of nickel or diffusion of nickel to the dielectric material at the step of firing a dielectric material in a high-temperature oxidizing atmosphere of 600° C. or higher; there is still difficulty in fabricating a high-capacity capacitor.
On the other hand, the outer surface (with no dielectric layer formed on it) of the metal foil for buried type capacitors must be coarsened to make sure contact strength for burying. As the dielectric layer is formed on the metal foil having a coarsened surface on one side, however, the influence of coarsening often passes onto the opposite surface, giving rise to the risk of making a short circuit in the device. After the formation of the dielectric layer, coarsening treatment may be carried out by etching, electroplating or electroless plating. However, this is found to bring about a high steps count and, in some cases, cause damage to the dielectric layer, again resulting in cost rises.
There is a transfer process for a previously prepared capacitor structure available, but it renders electrical connection of the capacitor structure to a capacitor electrode difficult. For the electrode provided prior to the formation of the dielectric layer, it is required to use costly materials such as platinum or gold for the purpose of heat resistance and oxidization resistance, and there is a soaring of costs for obtaining the thick electrode film of low electric resistance necessary as a capacitor well adapting to a high-frequency range.
In view of such situations, an object of the present invention is to provide a capacitor fabrication process capable of obtaining a thin-film capacitor at low costs and high yields that has a higher capacity and a form well suited as a buried type capacitor having a reduced overall thickness, and can be used even at high frequencies.